This application relates generally to condition responsive sensors and more particularly to combined fluid pressure and temperature sensor apparatus.
Fluid pressure responsive capacitive transducers comprising a variable capacitor mounted in a fluid chamber having a thin ceramic diaphragm exposed to the fluid so that changes in fluid pressure cause concomitant changes in the position of the diaphragm to thereby cause a change in the capacitance of the capacitor are well known in the art. Transducers of this type are shown and described, for example, in U.S. Pat. No. 4,716,492, the subject matter of which is incorporated herein by this reference. In that patent, a thin ceramic diaphragm is shown mounted in closely spaced, sealed, overlying relation to a ceramic substrate. Metal coatings are deposited on opposing surfaces of the diaphragm and base to serve as capacitive plates arranged in predetermined closely spaced relation to each other to form a capacitor. Capacitor terminal pins having one end connected to the capacitor plates are mounted in bores extending through the substrate with an opposite end connected to signal conditioning circuitry disposed in an electric circuit chamber at an opposite surface of the substrate. A connector body of electrical insulating material, received over the signal conditioning circuitry, mounts transducer terminals extending into the electric circuit chamber for connection to the signal conditioning circuitry. Capacitance is converted by the circuitry to an output voltage related to the fluid pressure. The capacitor is received in a suitable housing having a fluid receiving port and is provided with a fluid seal enabling the transducer to be used with fluid pressures up to 10,000 psi or higher.
In certain applications it is desirable to measure the temperature of fluid as well as pressure. Although it is known to provide temperature sensors in a separate or stand alone housing, it would be desirable to combine both functions in the same housing so that both measurements can be obtained at essentially the same location as well as to provide savings of cost and space. One of the problems in providing a device combining both functions is that of providing a seal between the electrical leads of the temperature sensing element and the interconnection with the electrical circuit means of the device in order to prevent leakage of the fluid being sensed, particularly at relatively high pressures, while still being able to accurately sense the temperature of the fluid within a satisfactory response time and without impairing the pressure sensing function. Sealing the temperature sensor by overmolding techniques addresses the leakage problem; however, it results in a relatively slow response time and an inherent temperature gradient between the medium being sensed and the sensing element. Sensors which are directly exposed to the fluid medium generally have an epoxy seal for the sensor leads; however, due to leakage problems, these are limited to use with relatively low pressure fluid systems.